1951 Pontiac Chieftain

New pressure regulator on the way.

i cant remember phil is this fundamentally the same as my 4 speed in the temptress ? good luck with the reg , i hope it works .

What's this?




It's been vacuumed, washed and waxed. Something's going on.

In this instance I believe it's not so much an issue that would be solved by cooling the oil; it's a thermal expansion problem, coupled with high mileage wear.
I had put the pressure gauge on it after changing the regulator, and at least now it modulates the main line pressure properly (it would not perform that relative to throttle position before, it would get hung up and keep the pressure low, then suddenly jump up as the piston became free).
Baseline is 60psi, with an increase to about 80psi when you've got your foot in it.

Selecting R brings it up 120-150psi (averaging about 135) which is correct.
The transition from Dr to N causes the line pressure to drop briefly to about 40psi.

What I saw when it started to act up was that it was correctly holding 60-80psi when hot, but at the point it decided to change gear after getting hung up, a brief drop (a fraction of a second) to about 40psi before resuming and holding 60 once the gear had engaged.

So, something is expanding at an inconsistent rate and getting sticky as it warms up. Reducing the oil temperature a little will likely not help a whole lot, as the valve body (alloy) is bolted to the transmission case (cast iron) and the shuttles, pistons and valves inside are all steel.

Something's out of tolerance but I cannot find what it is. The main pistons move freely, the shuttles were a little rough but cleaned up.. unless it's a portion of the way it works that I don't fully understand, I'm stumped.
Outside of taking it all apart again and sticking it in the oven to see what jams up, I'm out of ideas, hence looking for a replacement valve body.

Phil

In general the primary source of heating the oil in an automatic is from the shear forces in the torque converter. The fluid coupling in the Hydra-Matic doesn't have that issue, being direct coupled. Also, in top you're generally not pushing that much torque through the coupling.
Idling, the speed of the driving torus is reduced by being powered through the first epicyclic reduction gear.

They found that only when being used to run light armored vehicles, behind a 200+ horsepower V8 did the Hydra-Matic need a cooling system. And that was with old type A fluid.

The transmission doesn't get too hot that I cannot touch it after a drive.

A little extra-curricular reading tonight.



On my scratch paper, the band/clutch combination for each gear.

Note the DOUBLE TRANSITION VALVE (pointed to by pencil) which is meant to be triggered by the 2-3 valve.
What it does, when enabled, quickly dumps the front gearset into reduction (where it is for 1st gear). It overrides the gear selection valves.

So, if the DTV is stuck on, F is in reduction all the time.
In 1st that's not a problem, that's where it's supposed to be.
2nd is no change to the rear but swap the front to direct drive. If the DTV is stuck that won't happen; the selection from 1-2 keeps the gears in 1.
When 3rd is selected, the DTV should operate, dropping the front gearset back to reduction and putting the rear in direct.
Thunk, straight to third. It does that.
Then, fourth selected, no change until I snap the throttle a few times which causes enough pressure change to make 4th actually happen by shuffling the DTV closed again.

(If it sticks open, 1 and 2 are the same as 1, and 3 and 4 are the same as 3 on my paper, giving only 2 gears- at the shift point for 3rd)

Exactly what happens!

So, if all goes well I'll drop the transmission tomorrow and pull the DTV out (haven't pulled that out in a while because it's in the top of the valve block) and check for burrs, out of round, scores etc.

That should fix it. The reason the new regular made a difference is that the main line pressure now varies- that change seems to be about enough to get the DTV to shift back under spring pressure.

Phil

I've gotta fix and prove it yet, but the theory is sound.

what bit am i looking at ?

Again, drained and removed.


That valve there. Right there.


It's been dropped (like them all). Removed that burr and reassembled.

Test drive- cold, very good. Hot it now changes to 4th by itself but 1st gets hung up still, but can be made change by snapping the throttle shut above 5mph.

Improvement, for sure. I think the spring needs to be replaced, it's a bit weak.

Phil